J. S. Kristoff and D. F. Shriver
1788 Inorganic Chemistry, Vol, 12,No. 8, 1973 C5H5Fe(CO)?R (R = CH3 and CH2C6H5)do not react with CISOzNCO under comparable conditions. This shows that the formation of 21 from h5-C5H5Fe(C0)2CH2C(CH3)=CHz and CISOzNCO is almost certainly related to the allylic nature of the metal complex. In principle, this type of dualistic behavior with respect to rearrangement should also extend to V. However, a recent study on nucleophilic reactions of h5-C5H5Fe(CO)2(h5-CHz= C=CHR)+ (R = CH3 or C6H5)has demonstrated that the coordinated allene is rather inert to di~placement.~'Consistent with this observation, we note herein that rearrangement of the intermediates V invariably affords products containing a.A3-pyrrolinone ring (11). Registry No. C1S02NC0, 1189-71-5;h5-C5H5Fe(C0)2CH2CH=C(CH3)2, 38905-70-3;h5-C5H5Fe(C0)2CH2CH= CHC6H,, 3 1798-46-6;h5-C5H,Fe(CO)ZCH2CH=CHCH3, 40199-87-9; C6HSNH2,62-53-3;(CZH5)2NH, 109-89-7; (39) D. W. Lichtenberg and A. Wojcicki, J. Amer. Chem. SOC., 94, 8271 (1972).
C6H5COC1,98-88-4;h5-C5H5Fe(CO),CH2CtCCH3, 3482236-1; h5-C5H5Fe(C0)2CH2C=CC6H5, 331 14-75-9;MII(CO)~C H ~ C E C C ~ H23626-46-2; S, h5-C~H5Mo(C0)3CH2CSX6H5, 32877-62-6;h5-C5H5Fe(C0)2CH2C=CCHzFe(CO)2(h5-C5H5), 40199-91-5; CH30H, 67-56-1;NaBH4, 16940-66-2;Na[h5-C5H5Fe(C0),], 12152-20-4;C6H,CH0, 100-52-7;h5 C5H5Fe(C0)2CH2C(CH3)=CH2, 31781-60-9;p-CH3C6H4NH2, 106-49-0;Mn(C0)5CH2CH=CHC6H5,23108-58-9; 1,38599-33-6;2, 39015-07-1; 3,40196-20-1;4, 40196-21-2; 5,40196-22-3;6 , 40196-23-4; 7 , 40196-24-5; 8, 40196-25-6; 9,40196-26-7; 10,40196-27-8;11, 40196-28-9; 12,4019629-0; 13,40196-30-3;14,40196-31-4;15,40330-49-2;16, 40196-32-5; 17, 40196-33-6; 18,40196-34-7;19, 40187-040; 20,40187-05-1;21, 38905-68-9; 22, 38905-69-0; 23, 40 199-84-6 Acknowledgment. We wish to acknowledge support of this research by the National Science Foundation (Grant GP35647X and preceding grants).
Contribution from the Department of Chemistry, and Materials Research Center, Northwestern University, Evanston, Illinois 60201
Metal Complexes as Probes of Donor-Acceptor Interaction. Vibrational Spectra of (?T-C~H~)F~(CO)~(CNMX~) (M = B, X = H, F, C1, Br, or CH3; M = A1 or Ga, X = Cl or CH3) J. S . KRISTOFF and D. F. SHRIVER*
Received January 18, 1973 The change in the asymmetric CO stretching frequency, A u ~ ~ a s y which m, occurs upon coordination of an acid to (nC,H,)Fe(CO),(CN) provides a convenient probe for the electron pair acceptor strength of the acid. In contrast with previous attempts t o employ frequency shifts as probes of Lewis acids, the present method is not affected by spurious kinematic coupling between the probe oscillator and other oscillators in the adduct. Values of A u ~ ~ a s Ywere m determined for group 111 Lewis acids, and from these, relative electron pair acceptor strengths were deduced: BH, < BF, < BCl, = BBr,, BC1, > GaC1, > AlCl,, and Al(CH,), > B(CH,), = Ga(CH,),. Comparison of trends in electron pair acceptor strength with acidity trends yields valuable insight into the details of Lewis acid-base interaction. For example, such a comparison provides experimental evidence for the distortion energy of BH, being less than that of BF,.
Introduction Assessment of the extent of donor-acceptor electron transfer is a central problem in the understanding of bonding in complexes. While the relative acidity of an acceptor as judged by AG," or AH," is important in describing the extent of complex formation, these thermodynamic data usually do not provide specific information on the donor-acceptor bond because a variety of other bonds are altered upon complex formation. For example, the interaction of a donor with BF3 leads to considerable change in B-F bond lengths and angles, indicating significant changes in the B-F bond energies. More specific information on the donor-acceptor bond is available from interatomic distances and from force constants, both of which are difficult and sometimes impossible to determine with the required precision. These difficulties have prompted the use of simple spectroscopic criteria for the extent of donor-acceptor interaction, the foremost being nmr chemical shift and infrared absorption frequencies. However, these methods have serious limitations.' For (1) (a) T . D. Coyle and F. G. A. Stone, J. Amer. Chem. SOC., 8 3 , 4138 (1961); (b) R. C. Taylor, Advan. Chem. Ser., No. 42 (1964); (c) D. F. Shriver and B. Swanson, Inovg. Chem., 10,1354 (1971). (d) By contrast, a very promising spectroscopic probe for donoracceptor interaction involves the esr of free-radical bases; see T. B. Eames and B. M . Hoffman,$. Amer. Chem. SOC.,93, 3141 (1971).
example, the complex factors which determine nmr chemical shifts lead to considerable uncertainty in the application of this technique to a study of acceptor character." Infrared absorption frequencies associated with either the donor-acceptor bond stretch or with the stretch of an adjacent bond, e.g., vcN in nitrile adducts, are of limited value because these modes generally include large contributions from atom motions other than the ones of primary interest.'b3c In the present work we explore the use of shifts in vco as criteria for the extent of donor-acceptor interaction with the nitrogen end of cyanide in ( T - C ~ H ~ ) F ~ ( C O ) ~ ( C The N). attractive features of the metal carbonyl cyanide as a probe for donor-acceptor interaction are the sensitivity of CO stretching frequencies to the nature of the group attached to cyanide and negligible kinematic coupling of extraneous motions with the CO stretch. Experimental Section Materials. (n-C,H,)Fe(CO),CN was prepared by a modification of Piper, Cotton, and Wilkinson's procedure' in which the refluxing step was replaced b y stirring at room temperature for 1.5 hr (yield 40-50%). The compound was judged pure by C, H, and N analyses and infrared spectra. AlCl, (Matheson, Coleman and Bell) was ( 2 ) T. S. Piper, F. A. Cotton, and G. Wilkinson, J . Inorg. Nucl. Chem., 1, 165 (1955).
Vibrational Spectra of ( T - C ~ H ~ ) F ~ ( C O ) ~ ( C N M X ~ ) sublimed in vacuo at 125" before use. GaCl, was synthesized and purified by previously reported procedure^.^ BF,, BCl,, BBr,, B,H6, B(CH,),, and Ga(CH,), were purified b y trap-to-trap distillation until their vapor pressures agreed with literature value^.^ Trimethylaluminum (Ethyl Corp.) was purified by vacuum distillation. All solvents were distilled prior to use and were stored over Linde 4A molecular sieves. General Procedures. Except for aluminum trichloride and gallium trichloride, all Lewis acids were manipulated o n a preparative high vacuum line. Reaction of these acids with the metal compound was carried o u t in grease-free O-ring sealed equipment. For BF,, BCl,, B,H,, and B(CH,), the stoichiometry of interaction was followed b y PVT measurement of initial and unused Lewis acid, as well as weight gain of the solid. The GaC1, adduct was prepared in a grease-free vacuum line filtration apparatus., Aluminum trichloride complexes were prepared in standard Schlenk equipment. A nitrogen-filled drybox was used for solids transfer. Conductance of 7 X lo-, M (n€,H,)Fe(CO),(CNGaCl,) solution in nitrobenzene was measured in an air-tight cell having a cell constant of 0.919 cm-'. Analyses were performed b y Dornis and Kolbe Micro-Analytical Laboratories, Mulheim-Ruhr, West Germany, and in some cases were verified b y the Northwestern Analytical Services Laboratory. (n-C,H,)Fe(CO),(CNBF,). A tensimetric titration of 0.423 mmol of (n-C,H,)Fe(CO),CN suspended in ca. 1.5 ml of CH,Cl, a t 0" was carried o u t with BF,. A break in the titration curve occurred at 1:1.15 mole ratio of (n-C,H,)Fe(CO),CN:BF,, while weight gain indicated a ratio of 1:1.05. In a similar experiment with "BF, the mole ratio was 1:1.02 from PVT measurement of initial and unused BF,. I n b o t h cases the compound was pale yellow and air-sensitive. Anal. Calcd for (C,H,)Fe(CO),CNBF,: C, 35.48; H, 1.86; N, 5.19. Found: C, 35.42; H, 1.81; N, 5.20. (n-C,H,)Fe(CO),(CNBCl,). An excess of BCl, was added t o a suspension of 0.710 mmol of the iron compound in ea. 1.5 ml of chlorobenzene at 0". After 4 hr of stirring, excess BCl, was removed. Weight gain data indicated a ratio of 1:1.09 for (n-C,H,)Fe(CO),(CN):BCl, and in a similar b u t separate preparation a ratio of 1:1.07 was obtained b y PVT measurement of the initial and unconsumed BC1,. The product is off-white and appears to b e somewhat more stable t o air and hydrolysis than the BF, adduct. Anal. Calcd for (C,H,)Fe(CO),(CNBCl,): C, 30.01; H, 1.57; N, 4.37. Found: C, 31.61; H, 1.75; N, 4.68. (n-C,H,)Fe(CO),(CNBBr,). An excess of BBr, was added to a stirred suspension of 0.438 mmol of the parent iron compound in ca. 1.5 ml o f benzene. After 3 hr at ea. 0" excess BBr, and solvent were removed t o yield a pale yellow compound which is stable in air for short periods of time. From the weight gain t h e ratio of (n-C,H,)Fe(CO),(CN):BBr, was 1:1.01. Anal. Calcd for (C,H,)Fe(CO)(CNBBr,): C, 21.18; H, 1.11; N, 3.08. Found: C, 21.77; H, 0.97; N, 2.95. (n-C,H,)Fe(CO),(CNBH,). Excess B,H, was added to 0.429 mmol of the parent iron compound in 1.5 ml of chlorobenzene. After stirring for 8 hr, excess B,H6 was removed and determined by PVT measurement to yield an iron complex t o BH, ratio of 1:1.15. The compound is pale yellow and appears to be more stable toward hydrolysis and air than the boron halide adducts. Anal. Calcd for (C,H,)Fe(CO),(CNBH,): C, 44.31; H, 3.72; N, 6.46. Found: C, 43.90;H, 4.00; N, 5.85. (n-C,H,)Fe(CO),(CNAlCl,). Equal volumes of a 0.1 M nitrobenzene solution of (nC,H,)Fe(CO), (CN) and a nitrobenzene solution 0.1 M in AlCl, were mixed under an N, atmosphere and in the dark. The resulting mixture was directly examined by ir techniques. Attempts to isolate a crystalline adduct (in Schlenk equipment) by addition of low polarity solvents were unsuccessful. However, nmr spectra revealed a resonance due to parent (n-C, H,)Fe(CO),(CN) and one due t o the adduct for mole ratios of cyanide complex t o AlC1, greater than 1:1. At the 1 :1 stoichiometry a single adduct resonance is observed, which reverts t o the shift of parent cyanide complex upon introduction of excess triethylamine. (n-C,H,)Fe(CO),(CNGaCl,). T o 2 ml of diethyl ether containing 0.460 mmol of GaCl,, 0.410 mmol of (n-C,H,)Fe(CO),(CN) was added, and the suspension was stirred for 24 hr at 0". The light yellow chalky product was filtered off. Anal. Calcd for (C,H,)Fe(CO),(CNGaCl,): C, 25.34; H, 1.32; N, 3.71; C1, 28.05; Ga, 18.39. Found: C, 25.25; H, 1.80; N, 3.72; C1, 27.91; Ga, 18.26. (n-C, H, )Fe(CO), (CNAl(CH,),). T o a suspension of 0.572 mmol of (n-C,H,)Fe(CO),(CN) in 2 ml of xylene was added 2.5 mol of ( 3 ) W. C. Johnson and C. A. Haskew, Inorg. Syn., 1 , 2 6 ( 1 9 3 9 ) . ( 4 ) D. F. Shriver, "The Manipulation of Air-Sensitive Compounds," McGraw-Hill, New York, N. Y., 1 9 6 9 . ( 5 ) Reference 4 , p 101.
Inorganic Chemistry, Vol. 12, No. 8, 1973 1789 A K H , ) , . The pale yellow suspension gradually changed t o a yellowbrown solution. After 12 hr at O", xylene and excess Al(CH,), were removed leaving the yellow-brown compound. The determination of active methyl b y hydrolysis gave 1.05:1 for the (n-C,H,)Fe(CO),CNAl(CH,), ratio. (n-C,H,)Fe(CO),(CNB(CH,),). A suspension of 0.5 14 mmol of (n-C, H,)Fe(CO), (CN) in 2 ml of xylene was titrated tensimetrically with B(CH,), at 0". After each addition, a slight increase in pressure was noted, as well as a large increase in solubility. The titration indicated a B(CH,),/Fe ratio of 1.02. The yellow solution was allowed to warm to room temperature, followed b y separation of B(CH,), from xylene b y trap-to-trap fractionation. An infrared spectrum of the remaining solid was identical with that of the parent compound, demonstrating that the adduct readily loses B(CH,), under vacuum at room temperature. Samples used for infrared spectroscopy were prepared with a slight excess of trimethylboron, followed b y syringing the cold solution directly into the infrared cell. (n-C,H,)Fe(CO),(CNGa(CH,),). T o a suspension of 0.615 mmol of (n-C,H,)Fe(CO),CN in 2 ml of C6H6 was added an excess of Ga(CH,), . Upon addition, a yellow-brown solution resulted. After 5 hr at =lo",excess Ga(CH,), and benzene were removed under vacuum yielding a light-yellow chalky product. Anal. Calcd for (n-C,H,)Fe(CO),(CNGa(CH,),):C, 41.57; H, 4.44: N, 4.40; Ga, 21.93; Fe, 17.57. Found: C, 47.60; H, 4.86; N, 4.25; Ga, 21.43; Fe, 16.98. Spectra. All medium infrared spectra were obtained with a Beckman IR 9. CO and CN band positions were determined with slow scans on an expanded scale and 2-cm-' resolution. The reported values represent averages from several scans, with an observed reproducibility for solution spectra of k0.5 cm". Calibration of the instrument was periodically checked with polystyrene film. Solution spectra of the halide adducts were obtained with 0.05 M nitrobenzene solutions using 0.05-mm path length Irtran-2 windowed cells. Farinfrared spectra were obtained with a Beckman IR 11 o n Nujol mulls between polyethylene windows. For the trimethyl adducts infrared spectra were obtained under identical conditions for 0.02 M solutions of the adducts in toluene using 0.10-mm NaCl cells. Nmr spectra were obtained with a Varian T-60 o n 0.5 M samples in nitrobenzened , solution.
Results and Discussion Characterization of the Adducts. As with previous reports of Lewis acid interaction with carbonyl containing cyanides,6 the formation of adducts produced an increase in vCN of 30-80 cm-', characteristic of M-CN- acid linkages7 Furthermore, smaller increases in both CO stretching frequencies are observed in comparison t o the parent (n-CsH5)Fe(C0)2(CN), thereby eliminating the possibility of Fe-COacid linkages, Figure 1 and Table I. (Although M-CO- acid linkages have been reported: such interactions have always produced a decrease in vco of the C and 0 bonded CO and an increase in vco of the remaining groups of the parent compound.) The possibility of substitution on the C,H5 ring was eliminated by nmr spectra which yielded a single sharp H resonance for the 1: 1 adducts examined. All of the adducts with halide acceptors display M-X stretching frequencies in the region characteristic of four-coordinate group I11 metals (Table 11), an observation again consistent with simple molecular adducts, (n-CsHs)Fe(C0)2(CNMX3). For the GaC13 adduct, where the ionic structure [((n-CsHs)Fe(CO)2(CN))2MXz] + [MX4]- must be considered? (6) (a) R. B. King, Inorg. Chem., 6 , 25 ( 1 9 6 7 ) ; (b) P. J. C. Walker and R. J. Mawby, J. Chem. SOC.A, 3 0 0 6 (1 97 1 ) . (7) (a) D. F. Shriver, J. Amer. Chem. Soc., 8 5 , 1405 ( 1 9 6 3 ) ; (b) D. F. Shriver and J. Posner, ibid, 88, 1672 ( 1 9 6 6 ) ; (c) J. J. Rupp and D. F. Shriver, Inorg. Chem., 6 , 755 (1967); (d) D. Hall, J. H. Slater, B. W. Fitzsimmons, and K. Wade, J. Chem. SOC.A , 8 0 0 ( 1 9 7 1 ) . (8) (a) N. J. Nelson, N. E. Kime, and D. F. Shriver, J. Amer. Chem. SOC.,9 1 , 5 1 7 3 ( 1 9 6 9 ) ; (b) J. C. Kotz and C. D. Turnipseed, Chem. Commun., 41 ( 1 9 7 0 ) ; (c) J. M. Burlitch and R. B. Petersen, J. Orgunometul. Chem., 2 4 , C65 ( 1 9 7 0 ) ; (d) A. Alich, N. J. Nelson, and D. F. Shriver, Chem. Commun., 2 5 4 ( 1 9 7 1 ) ; (e) R. B. Petersen, J. J. Stezowski, C. Wan, J. M. Burlitch, and R. E. Hughes, J . Amer. Chem. SOC.,9 3 , 3532 ( 1 9 7 1 ) ; (f) D. F. Shriver and A. Alich, Coord. Chem.
Rev., 8, 15 (1972). (9) C. D. Schmulbach and I . Y. Ahmed, Inorg. Chem., 1 0 , 1 9 0 2 (1971).
1790 Inorganic Chemistry, Vol. 12, No. 8, 1973
J. S. Kristoff and D. F. Shriver
Table I. CO and CN StretchingFrequencies (cm-') for (n-C,H,)Fe(CO), (CN) Adductsa VC N
Adduct Parentb AlCl, BF3 "BF, BCl,
BBr, BH, GaC1, ParentC B(CH,), Al(CH,), Ga(CH,),
VCOasYm
,COSY*
Mull
Soln
2117.4 2172.1 2195.9 2196.5 2205.4 2190.5 2194.2 2151.7
2121.0 2168.1 2201.0 2201.2 2191.4 2182.5
Mull 2058 2063.7 2076.0 2077.5 2071.2 2071.0 2070 2072.4
2152.1 2026.5 2194.5 2164.9 2166.2 2068.9 2155.8 2157.3 -2058
Soln 2054.7 2068.3 2073.0 2073.3 2076.5 2077.0 2073.2 2054.5 2066.1 2069.1 2064.7
Mull 2000.0 2014.5 -2025 -2025 2023.7 2026.6 2016.9 2034.0
Soln 2006.9 2025.9 2030.8 2031.1 2037.4 2038.9
2035.1 2008.2 2023.1 2032.3 2027.8 2012.2 2022.0
a Solution spectra of the halide adducts were obtained on 0.05 M solutions in nitrobenzene. For the group I11 trimethyl adducts 0.02 M solutions in toluene were employed. b Solution spectra obtained using nitrobenzene as solvent. C Solution spectra obtained using toluene as solvent.
Table 11. Far-Infrared Data Compound (n-C, H, )Fe(CO),CN (n-C,H,)Fe(CO),(CNBF,) (n-C,H,)Fe(CO),(CNBCl,) (n-C,H,)Fe(CO),(CNBBr,)
Frequencies, cm-' 358 mw, 379 m, 410 m, 462 m, 482 w, 501 mw, 520 m, 561 s, 599 s, 607 s: 614 vs 343 w, 375 mw, 390 mw, 402 m, 442 w, 489 m, 507 m, 525 m, 546 mw, 568 vs, 596 vs, 598 sh, 608 sh 374 mw, 406 w, 418 m, 468 w, 493 m, 518 vs, 561 vs, 594 vs, b, 600 s, 605 s 290 mw, 371 w, 400 w sh, 404 mw, 451 mw, 475 mw, 491 m, 514 s, 559 vs, 588 vs, 596 m, 603 m
the molar conductance for a 7 X M solution in nitrobenzene was found to be 1.64 ohm-' cm-2. This value is much lower than the molar conductance obtained for a similar tetraphenylarsonium chloride solution (18.7 ohm-' cm-') as well as values reported for known 1: 1 electrolytes in nitrobenzene," indicating that the molecular GaC13 adduct is the predominant, if not exclusive form in nitrobenzene solution. Shifts in vco . It is well documented that an increase in vco occurs upon a decrease of electron availability from other ligands in a mixed carbonyl complex. For example, the carbonyl stretching frequencies of LW(CO)5 complexes increase as the basicity of L decreases.""' There is some difference of opinion on the detailed interpretation of this effect. Some workers either assume or conclude that back 77 bonding to L is the primary mechanism by which vco is increased,l3>I4while others attribute this increase in vco among a series of substituted carbonyls to both decreases in u donation and'increases in back 71 bonding by L.15 In addition, recent molecular orbital calculations provide evidence for the perturbation of the CO bond by direct donation from L to cis carbonyls." Fortunately, all of these mechanisms should give rise to the same trend in predicted shift so their relative importance does not have t o be resolved for the interpretation of (n-C,H5)Fe(C0)2(CNMX3)carbonyl stretching frequencies. Upon coordination of a Lewis acid to the nitrogen end of a metal cyanide complex, both spectroscopic evidence and molecular orbital calculations reveal that the cyanide in a metal complex becomes a potentially poorer u donor and better 71 acceptor.7b Therefore, within any of the current interpretations of CO stretching frequencies for substituted carbonyls, vco should increase with increased electron pair acceptance by a Lewis acid attached to the cyano nitrogen. As described in the Introduction, most previous correlations between vibrational frequencies and donor-acceptor interaction are highly suspect owing to the inadequacy of a localized description for the modes of interest. The question naturally arises as to whether the trends observed for vco of (n-CSH5)Fe(C0)2(CN)adducts are substantially influenced by coupling with other coordinates in the adducts. To provide a quantitative answer to this question, Gvco is defined as the change occurring in vco upon adduct formation due entirely to changes in frequency of other oscillators within the molecules which are coupled to the CO oscillator. The application of perturbation methods to the problem of a system of oscillators coupled t o the CO oscillator yields a value for the CO frequency parameter XCO given in eq 1.17
In the present case the summation in eq 1 extends over all (10) (a) W. J . Geary, Coord. Chem. Rev., 7 , 8 1 (1971); (b) J . Selbin and L. H. Holmes, Jr., J. Inorg. Nucl. Chem., 24. 1111 (1962). (1 1) R. J. Angelici and M. D. Malone, Inorg. Chem., 6, 1 7 3 1 (1967). (12)
For a review of many similar correlations, see L. M. Haines and M. H. B. Stiddard, Advan. Inorg. Chem. Radiochem., 12, 53
Figure 1. Infrared spectra of A, (n-C,H,)Fe(CO),(CN), B, (nC,H,)Fe(CO),(CNBF,), and C, (n-C,H,)Fe(CO),(CNBCl?). The 2000- and 2200-cm-' positions are indicated by &tical lines below each spectrum. Nujol peaks are indicated by +.
(1969). (13) W. D. Horrocks, Jr., and R. C. Taylor, Inorg. Chem., 2 , 7 2 3 (1963). (14) F. A. Cotton, Inorg. Chem., 3, 702 (1964). ( 1 5 ) D. J . Darensbourg and T. L. Brown, Inorg. Chem., 7 , 959 (1968). (16) (a) R. L. DeKock, A. C. Sarapu, and R. F. Fenske, Inorg. Chem., 10, 38 (1971); (b) R. F. Fenske and R. L. DeKock, ibid., 9, 1053 (1970). (17) E. B. Wilson, Jr., J . C. Decius, and P. C . Cross, "Molecular Vibrations," McGraw-Hill, N e w York, N. Y., 1955, p 2 2 9 .
Vibrational Spectra of ( T - C ~ H ~ ) F ~ ( C ~ ) ~ ( C N M X ~ ) the stretching and deformation coordinates of either A' or A" symmetry for the Fe(C0)2CN moiety, exclusive of the CO coordinate, for which i = 1.lSa To a good approximation, both Hii and Hll in the denominator can be replaced by the corresponding observed frequency parameters Xi and AI. Utilizing the observed hco for the parent compound, HI, was determined. Then, using primed quantities for the adduct and unprimed for the parent, Fvco is given by eq 2.
Off-diagonal matrix elements in eq 2 are calculated from the product of the kinetic energy and force constant matrices, H = GF. The G matrix was calculated'8b using bond distances and angles from closely related molecules: l9 L(OCFeC0) = L(NCFeC0) = 90" ;r(Fe-C) = 1.76 r(C0) = 1.14 A. The F matrix was constructed from a list of typical force constants given by Jones for M(CO), species." (This procedure is reasonable in view of the transferability of interaction constants from M(CO), to Fe(CO),.") When a range of force constants was quoted, the extreme values were used which led to the greatest influence in Gvco. Another measure taken to assure that Gvco would not be underestimated was the assumption that force constants involving CN are the same as those for CO. The transformation from an internal to a symmetry coordinate basis yields two blocks each for F, and G,, one 9 X 9 for A' coordinates and one 6 X 6 for A" coordinates. The corresponding H matrix for the A' block contains nonzero values for all matrix elements of the type Hli and Hi,. Therefore by eq 2, shifts in all of the symmetric Fe(C0)2(CN) modes will influence the symmetric CO stretch, but, judging from the magnitude of these elements, most of the symmetric modes will exert relatively little influence on v C 0 w m . As had been anticipated, mixing is much less extensive in the A" block with all products HilHli (i = 3,4, . . ., 6) equal to zero; consequently, eq 1leads to the conclusion that the asymmetric CO stretch is only influenced by the asymmetric FeC stretch (i = 2). Previous studies by Wade and coworkers have shown that vFecis relatively insensitive to adduct formation on the cyano n i t r ~ g e n . ' ~For example, the TlU FeC stretch shifts from 417 cm-' in K4[Fe(CN)J to 422 cm-' in K4[Fe(CNBF3),]. Judging from Manning's assignments for (nC5H5)Fe(C0)2Xcompounds,21 a 462-cm-' absorption for (n-C5H5)Fe(CO),(CN) is most likely the asymmetric Fe-CO stretch. This band shifts to 442,468, and 45 1 cm-' for the BF3, Bel3, and BBr3 adducts, respectively. From eq 2 these shifts lead to very small kinematic perturbations of vco : Gvco = $0.04, -0.01, +0.02 cm-l, respectively, for BF3,
a;
.
(1 8) (a) The BX, coordinates are not included in this summation as second-order perturbation theory and simple physical reasoning shows that direct kinematic interaction of BX, and CO coordinates will be nil. Also, it is reasonably assumed that negligible changes will occur in the vcyclopentadiene-metal vibrations and therefore coordinates involving the ring are neglected. (b) Program GMAT: J. H. Schachtschneider, "Vibrational Analysis of Polyatomic Molecules V,Ia Report 231-64, Shell Development Co., Emeryville, Calif. (19) (a) P. T. Green and R. F. Bryan, J. Chem. Soc. A , 1696, 2261 (1970); (b) D. F. Shriver, Stvuct. Bonding (Berlin), 1, 32 (1966). (20) L. H. Jones, "Inorganic Vibrational Spectroscopy," Vol. I , Marcel Dekker, New York, N. Y., 1971, pp 1 5 0 , 157. (21) A. R. Manning,J. Chem. SOC.A , 1670 (1968).
Inorganic Chemistry, Vol. 12, No. 8, 1973 1791 BC13, and BBr3 adducts. Even if this particular assignment is incorrect, the small shift (GaC13 (28 cm-') > AlC13 (1 9 cm-l). However, the group I11 trimethyl derivatives show a much smaller variation and different order: Al(CH& (20 cm-') > B(CH3)3 (15 cm-') Ga(CH3)3 (14 cm-'). Comparison of Electron Pair Acceptor Strengths with Acidities. We have consistently referred to the quantity being measured by A v C O ~ m as the relative electron pair acceptor strength and not the relative acidity. While the point has been made previously,'ci23 it is worth stressing that spectroscopic probes in general measure the relative strength of the donor-acceptor bond but not the acidity. The term acidity, in our view, is best reserved for thermodynamic data such as AH" or AGO of complex formation. Even when the
-
-
-
(22) B. Swanson, D. F. Shriver, and J. A. Ibers, Inovg. Chem., 8 , 2182 (1969). (23) J . M . Miller and M. Onyszchuk, Can. J. Chem., 43, 1873 (1965).
1792 Inorganic Chemistry, Vol. 12, No. 8,1973
J. S . Kristoff and D. F. Shriver
thermodynamic data is corrected for possible variations in physical state and degree of association of the acid (e.g., B& + 2BH3), the resulting acidities will not necessarily parallel the electron pair acceptor strengths. The reason lies in the fact that, in contrast with the spectral probe which provides a measure of the donor-acceptor bond interaction, the enthalpy or free energy of reaction includes contributions from changes in other bonds as well. The classic example is provided by the reorganization of the boron halides from planar triangular with short B-X bonds in the free state to a pyramidal geometry with longer B-X bonds in the adduct, eq 3. I t is clear from this cycle that the heat of com-
+
D BX, pyramidal R ./
\
m D B
(3)
D: +,BX, - - - - - - + D : B X , planar AHc
plex formation, AHc, will be less than the donor-acceptor bond enthalpy, A H D , , owing to the endothermic process of reorganization, AHR. Thus, comparison of acidities (=4HC) with electron pair acceptor strengths (-AHDA) will, in certain cases, provide information on the reorganization processes (AHR). A good illustration of such an influence is provided by comparison of heats of complex formation of BH3 and BF3 toward a nitrogen donor, eq 4.’“ The similarity in these BH3(g)
+ N(CH,),(d
= H3BN(CH3)3(g)
AH,”= -31.3 kcal/mol BF3(g) + N(CH,),(g) = F,B”I,),(g)
(4)
AHc” = -30.9 kcal/mol heats of reaction is to be contrasted with the order of electron pair acceptor strengths, BF3 > BH3, found in the present work. This comparison provides direct experimental evidence that AHR is more positive for BF3 than for BH,. Therefore, BH3 is the easier of the two to distort, in agreement with the reorganization energy estimates of Parry and Alton.” The relative electron pair acceptor strengths for the group 111 chlorides, BC13, AlC13, and GaC13, are of particular interest because of the frequent application of these acids as catalysts. Satchell and Satchell have argued from the relative radii of B, Al, and Ga that there should be a monotonic decrease in the affinity for hard bases in the order B > AI ,> Ga,26 and enthalrq data in a coordinating solvent tend to support this order. Toward pyridine as a reference base, the enthalpy data indicate a different order of acidity2’ AlC13 > BCI3 > GaC13. While the position of BC13 relative to GaC1, may be somewhat variable. there is agreement that BC1, is the stronger acid of the two. Given this higher acidity of BC13 (more negative AHc) and the expected higher reorganization energy of BC13 (more positive A H R ) , it follows from eq 3 that the donor-acceptor bond energy should be greater for (24) C. T. Mortimer, “Reaction Heats and Bond Strengths,” Pergamon, London, 1962, p 122. ( 2 5 ) R. W. Parry in “Coordination Chemistry,” S. Kirschner Ed., Plenum Press, New York, N. Y., 1969, p 207. Ideally we should compare relative heats of complex formation with electron pair acceptor strengths for the same base, (n-C,H,)Fe(CO),(CN) in the present case. A potential complication in our interpretation here is that F-CH, steric interaction makes a significant positive contribution to AH,’ for F,BN(CH,),. (26) D. P. N. Satchell and R. S. Satchell, Quart. Rev., Chem. SOC.,
BC13 (more negative AHDB) than for GaC13 adduct formation. This conclusion agrees with the relative electron pair acceptor strengths which were determined for BC13 and GaC13 in the present work. The unusually low electron pair acceptor strength for AlC13, below that of GaCl,, does not have a ready explanation. We have, however, confirmed that this order is paralleled by relative affinities for (T-C~H,)F~(CO)~(CN) by a competition experiment in which a stoichiometric quantity of AlC13 in nitrobenzene was added to a nitrobenzene solution of (nC5HS)Fe(C0)2(CNGaC13). Judging from vco .GaC1, was not displaced. It will be recalled that the AlC13 adduct could not be isolated as a solid and therefore was not characterized as completely as the other trihalide adducts. It is possible that unlike the other members of this series, the AlC13 complex is not a simple MX3 adduct. Two reasonable alternate formulations are an ionic complex [((n-C5H5)~Fe(C0)2 (CN))2AlC12+] [AlC14-] or a solvent-coordinated species (n-
CSHS)Fe(C0)2(CN)A1C1302NC6HS. The enthalpies of formation for trimethylamine adducts of B(CH3) IU(CH,)~, and Ga(CH3)3 demonstrate the acidity order Al > Ga ,> B,29 which is to be compared with the order of electron pair acceptor strength, AI > B = Ga, based on A v c o w m . The rather small change in position for trimethylboron in these two series may result from the smaller steric hindrance to trimethylboron complex formation with (n-CSHS)Fe(C0)2(CN)than with N(CH3).30 In line with this argument B(CH3), and Ga(CH3)3 are equally acidic toward P(CH3)3 which is less sterically encumbered than N(CH3)3.29d’30
Conclusion Relative values of A v C O w m for Lewis acid adducts of (nC,HS)Fe(C0)2(CN) form a convenient and reliable guide to trends in electron pair acceptor strengths of acids. In addition to convenience, the probe has the advantage that it can be applied to studies on the influences of different acceptor atoms (e.g., BC13 vs. GaC13) as well as studies on the influence of ligands in a single acceptor (e.g., BF3 vs. BC13). By contrast, bond length and force constant variations for the donor-acceptor bond provide information on the relative electron pair acceptor strengths only when comparison is being made between acids having the same acceptor atom. While the present work was confined to group I11 acceptors, (T~-C~H,)F~(CO)~(CN) or similar metal carbonyl cyanides should be generally applicable probes for main group and transition metal acceptor molecules as well as acceptor sites on surfaces. Fruitful comparisons can be made between electron pair acceptor strengths measured by spectral probes and acidities measured by thermochemical data. In general, electron pair acceptor strengths will parallel acidities because the donoracceptor bond strength usually dominates the heat of adduct formation. However, where the two are not parallel, the disparities provide good indications of factors other than the donor-acceptor bond enthalpy which contribute to the enthalpy of complex formation. One such factor which is difficult, if not impossible, to assess by any other means is the energy necessary to distort the acid to the geometry it possesses in the complex (reorganization energy).
2 5 , 171 (1971).
(27) D. P. N. Satchell and R. S. Satchell, Chem. Rev., 6 9 , 25 1 (1969). (28) ( a ) J . W. Wilson, and I. J . Worrall, J. Chem. SOC.A , 316 (1968); (b) N. N. Greenwood and T. S. Srivastava, ibid., 267 (1966); (c) J . D. Beck, R. H. Wood, and N. N. Greenwood, Inorg. Chem., 9, 86 (1970).
(29) (a) H. C . Brown and M. D. Taylor, J. Amer. Chem. SOC.,69, 1332 (1947); (b) H. C. Brown and N. Davidson, ibid., 64, 316 (1942); (c) G. E. Coates and N . D. Huck, J. Chem. SOC.,4501 (1952); (d) G. E. Coates, ibid., 2003 (1951). (30) H. C. Brown, J. Ckem. SOC., 1248 (1956).
Inorganic Chemistry, VoZ. 12, No. 8, 1973 1793
Dicyanobis(diimine)iron(III) Compounds
Registry No. (n-C5H,)Fe(C0)2(CNBF3), 401 99-93-7; BC13, 10294-34-5;BBr3, 10294-33-4; B2H6, 19287-45-7; (n-C5H5)Fe(C0)2(CNBC13), 40199-94-8; (n-C,H,)FeAlC13, 7446-70-0; GaC13, 13450-90-3;Al(CH3)3, 75-24-1; (C0)2(CNBBr3), 40199-95-9;(T-C,H,)F~(CO)~(CNBH~), B(CH3)3, 593-90-8; Ga(CH3)3, 1445-79-0. 40199-96-0;( T - C ~ H , ) F ~ ( C O ) ~ ( C N A ~40199-97-1 C~~), ; (n-C5H5)Fe(C0)2(CNGaC13),40330-5 1-6; ( T - C ~ H ~ ) F ~ ( C- O ) ~ Acknowledgment. This research was supported by the NSF through Grant GP-28878 and by the Northwestern (CNAl(CH,),), 40199-98-2;(n-CSH,)Fe(CO)z(CNB(CH3)3), 40199-99-3; (n-C5HS)Fe(CO)2(CO)2(CNGa(CH3)3), 40200University Materials Research Center. J. Kristoff acknowledges the receipt of an NDEA Fellowship. 00-8; (T-C~H,)F~(CO)~CN, 12152-37-3; BF3, 7637-07-2;
Contribution from the Departments of Chemistry, Northeastern University, Boston, Massachusetts 021 15, and Wayne State University, Detroit, Michigan 48202
Electronic Ground States of Dicyanobis(diimine)ion(III) Compounds W. M. REIFF*Ia and R. E. DeSIMONEIb Received January 12, 1973 Epr, nmr, and magnetically perturbed Mossbauer spectral data for Fe(phenanthroline),(CN),+ and Fe(bipyridine),(CN),+ are compared to similar data for the corresponding tris(diimine) systems. The principal component of the electric field gradient (V,,) is positive for all of these compounds and consistent with an orbital ground state corresponding t o a hole in d,2. Fits to epr spectra in various media are discussed and an axial (trigonal) model with splittings of -1000 cm-' in the ground *T,term is favored; z.e., the basic trigonal field of the tris(diimine) systems appears t o persist in the bis analogs. This is also supported by proton nmr data. Perturbed Mossbauer results for the low-spin ferrous systems [Fe(bipyridine),](ClO,), (V,, negative) and [Fe(bipyridine),(CN),] (Vzzpositive) axe considered in relation to the magnitude of the quadrupole splittings of the corresponding ferric systems.
Introduction The electronic structure of low-spin iron(lI1) complexes has been the subject of several recent investigations. Much of the effort expended toward the understanding of these complexes has focused on the more symmetric molecules of the ty e M(chelate)3"+. In particular, studies of the Fe(NN)!' complexes* have been most fruitful while investigations of sulfur-donor compounds such as Fe(dithioacety1acetonate), ,3-5 though providing equally interesting results, have proven much more difficult to interpret in terms of a unified model of bonding. In this paper we present the results of an nmr, epr, and magnetically perturbed Mossbauer study of the low-symmetry ferric complexes [Fe(diimine)?(CN),]X [diimine = 1,lo-phenanthroline(phen) or 2,2'bipyridine (bipy)] as well as the perturbed Mossbauer data for the low-spin ferrous analogs. The reduction in symmetry resulting from substitution of two CN- ligands for a chelating diimine of a tris(diimine) complex is expected to lower the symmetry to less than threefold, the highest remaining symmetry axis being the Cz which bisects the CN-M-CN angle. What is not apparent is whether this has a significant effect on the dominant trigonal distortion imposed by the chelate rings and, if so, to what factors the distortion is most sensitive. It is with these problems that the present study is concerned. Experimental Section The Mossbauer apparatus has been described previously.6 Magnetically perturbed Mossbauer spectra were determined at ambient temperature using a Varian Associates Model 4500 magnet (transverse geometry) and accompanying power supply. The applied (1) (a) Northeastern University. (b) Wayne State University. ( 2 ) (a) R. E. DeSimone and R. S . Drago, J. Amev. Chem. SOC., 9 2 , 2 3 4 3 ( 1 9 7 0 ) ; (b) W. M. Reiff, Chem. Phys. Lett., 8 , 2 9 7 ( 1 9 7 1 ) . (3) R. L . Martin and I. A . G. Roos, Aust. J. Chem., 24, 2 2 3 1 ( 1 9 7 1). (4) W. M. Reiff, Chem. Phys. Lett., 17, 2 8 8 (1972). (5) S. A. Cotton and J. F. Gibson, J. Ckem. SOC.A , 8 0 3 ( 1 9 7 1 ) . ( 6 ) W. M. Reiff, J. Chem. Phys., 5 4 , 4 7 1 8 ( 1 9 7 1 ) .
fields for cylindrical, tapered pole faces at 0.5-in. gap were nominally of the order of 20 kG,while the y-ray source (50 mCi 57C0in Cu) was maintained in a fringing field of less than 300 G . Nmr spectra were recorded on a Jeolco C-60H nmr spectrometer at 30' in DMSO-d,.' No evidence for coordination of DMSO was indicated. Spectra were calibrated relative to tetramethylsilane as internal standard. Epr spectra were obtained on a Varian E-4 spectrometer system at X-band frequency. Samples of ihe bis complexes were prepared as dilute solutions in N,N-dimethylacetamide-H,O and frozen rapidly in liquid nitrogen before reduction to the ferrous complexes could occur. Spectra were run in helium-cooled liquid N, at -73°K in a standard low-temperature dewar. Calibration was relative to diphenylpicrylhydrazyl (DPPH), g = 2.0036. All complexes were prepared as outlined in the literature.*
Results and Discussion Nmr. Nmr spectra of [Fe(phen)2(CN)2]Cand [ F e ( b i ~ y ) ~ (CN)2]+in DMSO-d6 clearly show two sets of nonequivalent protons due to the inequality of the chelate rings and unambiguously identify the compounds as the cis isomers (Figure 1). In the bipyridine complex, only the 6,6' resonance remains degenerate, while in the phenanthroline complex, both the 2,9 and 5,6 proton resonances remain degenerate as in the tris complex (Table I).' Interestingly, if one averages the two inequivalent resonances in the bis complexes and compares the result to the value found in the tris complexes, the difference is generally quite small. This would indicate that there is no great change in the net amount of spin density reaching the protons as one goes from tris to bis complexes. One has a situation where due to the CN-ligands, less spin density is placed on one half of the diimine and more on the other half. While the situation involving spin delocalization is complicated by several unknown factors, and is undoubtedly exceedingly complex, it seems reasonable to conclude that lowering of the symmetry in these bis complexes takes place without gross distortion of the basic symmetry of the tris complexes. Were this not (7) R. E. DeSimone, Ph.D. Thesis, University of Illinois, 1 9 7 0 . (8) A. A. Schilt, J. Amer. Ckem. SOC.,8 2 , 3000 (1960).
